1. Field of the Invention
This invention relates to a microstrip line type resonator, and more particularly, to a microstrip line type resonator structure including a rectangular parallelepiped dielectric plate, a ground conductor layer formed on the bottom, front, rear, and two side surfaces of the plate, and a microstrip line formed on the top surface of the plate.
2. Brief Description of the Related Art
Recently, high frequency microwave band communication technology has played a significant role in mobile communication systems, for example, in the recently developed cellular telephone systems.
In this technology, since communication systems require several hundreds of frequency channels in the approximately 800 MHz frequency band, there has been a need for a small filter, having a high quality factor (high-Q) and a low parasitic capacitance, which is suitable for mass production.
To realize such a filter, a small, high-Q resonator, which is an essential part of the filter, is required. One example of a conventional resonator is disclosed in a U.S. Pat. No. 4,266,206 issued on May 5, 1981.
FIG. 1 illustrates an example of the conventional microstrip line type resonator disclosed in the above mentioned U.S. patent. As shown in FIG. 1, the resonator 10 includes a rectangular dielectric substrate 12 which may be made of ceramic, such as alumina, having a thickness on the order of 0.03 inch. Further, the resonator 10 includes a ground plane conductor 14 on the bottom surface of the substrate 12, a microstrip line 16 on the top surface of the substrate 12, an apron portion 18 provided on the top surface connected to the microstrip line 16 at one end 22 of the microstrip line 16, and a conducting bridge 20 connecting the microstrip line 16 and the ground plane conductor 14 via the apron 18. Dielectric material of the substrate 12 is exposed at a portion of the top surface and two side surface 13a and 13b thereof.
Generally, this kind of resonator is designed by approximation calculations, and further, is adjusted by trimming based on repeated trial and error of actual measurements of several characteristics of the resonator, such as the quality factor, resonance frequency, and the like.
Publications have been directed to such approximation, some thesis or books have, for example, Wheeler, H. A., "Transmission Line Properties of a Strip on a Dielectric Sheet on a Plane", IEEE Trans. Microwave Theory Tech., Vol. MTT-25, August 1977, pp. 631-647, or Wheeler, H. A., "Transmission Line Properties of a Stripline Between Parallel Planes", IEEE Trans. Microwave Theory Tech., Vol. MTT-26, November 1978, pp. 866-876.
However, since these approximations are based on an ideal model such as that illustrated in FIG. 2 which includes a dielectric plate 28 having infinite area, a grounding conductor 26 provided on the entire bottom surface of the dielectric plate 28, and a microstrip line 24 having an infinite length on the top surface of the dielectric plate 28, the final adjustments must still be conducted to a complete the resonator, which has a limited area, to account for the differences between the ideal resonator model and the actual resonator structure.
Further, to enhance the approximation, the conventional resonators should have similar structure to that of the ideal resonator. On the other hand, since the resonator should be compact for use in high frequency filters featured in mobile telecommunication systems. Further, the electromagnetic fields between the microstrip line and the ground conductor, such as those illustrated as arrow lines (electric field) and broken lines (magnetic field) in FIG. 2, are distrubed by the limited area of the dielectric plate.
In other words, there must be such disturbance of the electromagnetic field in the actual resonator which has at least an exposed dielectric plane at the end of the dielectric plate, for example, the side surface 13a and 13b shown in FIG. 1. Further, the disturbed electromagnetic fields reduced the quality factor of the resonator.
Therefore, even though it has long been a need for a compact microstrip line type resonator having a high quality factor, the conventional microstrip line type resonator can not realize the desired (High-Q) resonator characteristic.